Gas turbine engines produce power by burning fuel in the presence of compressed air and then directing the resulting combustion gases through rotatable arrays of turbine blades. Each rotatable array of turbine blades is preceded by a non-rotatable array of blades which directs the combustion gases into the rotatable array of blades at an optimum angle. The preferred angle of impingement of the combustion gases on the rotatable array of blades is a function of the rotational speed and/or power to be produced by the engine.
The efficiency of the gas turbine engine also is determined by the proportion of the combustion gases that actually perform work by impinging upon the rotating arrays of blades, or rotors. More particularly, some of the combustion gases will pass through the gaps between the stationary and moving parts. These gaps must be present in view of the different expansion characteristics exhibited by the various rotating and non-rotating parts of the engine. Considerable work has been done to minimize the tip losses, or losses which occur due to the flow of combustion gases between the tips of the rotor blades and the adjacent non-rotating structures. For example, the prior art has included honeycomb seals mounted on either the rotating or non-rotating member. The cells of the honeycomb structure are aligned in a generally radial direction with respect to the rotational axis of the turbine engine. It has been found that the presence of this honeycomb structure creates a turbulent air flow which cuts down on tip losses. Similarly, some turbine engines include knife-edge seals formed from generally annular rings mounted in offset relationship to both the rotating and non-rotating members of the engine. These offset annular members define a convoluted path through which air must travel, thereby creating localized pressure drops and turbulence which again minimize tip losses.
As noted above, efficiency is also affected by the angle at which the combustion gases impinge upon the turbine rotors. The angle of impingement for obtaining optimum efficiency will vary in accordance with the operating speed of the engine. Gas turbine engines that are employed in aircraft operate at or near the maximum speed most of the time. Consequently, for most operating conditions, there will be little variation in the optimum angle of impingement of the combustion gases on the rotor blades. As a result, the stator blades are fixedly mounted in the engine to direct the combustion gases toward the rotor at an angle that will achieve optimum efficiency during high power engine operating conditions.
Gas turbine engines provide a power to weight ratio that is desirable for many ground vehicles. Consequently, gas turbine engines are being used with increased frequency in certain ground vehicles, such as armored tanks, armored personnel carriers and trucks. The use of these engines in still other vehicles is possible. Much of the gas turbine technology developed for aircraft can be applied directly to ground vehicles. However, ground vehicles are likely to operate over a much broader range of engine speeds. Thus, in contrast to aircraft, ground vehicles are likely to spend a relatively large proportion of their time idling or at relatively low engine operating speeds. However, ground vehicles frequently will be called upon to operate efficiently at high engine speeds. In view of this wide range of engine operating conditions, some turbine powered ground vehicles employ variable stators which enable selective adjustments to the angles at which the combustion gases impinge upon the blades. More particularly, each stator blade is operative to selectively move about its own centerline which in turn extends substantially along or parallel to a radial line of the turbine engine. The stator blades will be adjusted in accordance with the engine operating speed in an effort to maximize fuel efficiency. Generally, at high engine speeds, the stator blades will be opened to permit a more direct axially aligned flow of combustion gases. At lower engine speeds, the stator blades will be closed somewhat.
The opposed radial ends of each variable stator blade include generally radially aligned pins about which the respective stator blades may pivot. At least one of the pins typically will include an appropriate structural means to effect the movement of the stator blade. For example, one pin of each stator blade may be provided with an array of gear teeth. The gear teeth may be mechanically interconected with other operable parts of the engine to effect the adjustments to the stator blade alignment with varying engine speeds.
The opposed radially inward and outward tips of each variable stator blade must be spaced from the stationary supports of the engine to enable these adjustments and to account for differential rates of thermal expansion. Typically, the sum of the radially inner and radially outer gaps will equal approximately 4% of the total radially extending space within which the variable stator is disposed. These radially inner and outer gaps can result in substantial tip losses, with a corresponding detrimental effect on engine operating efficiency. Thus, the improvements in efficiency that are attained by employing variable stators in a gas turbine engine are at least partly offset by the tip losses that result from the prior art variable stator blade.
The seals that have been developed in connection with turbine rotors can be applied to variable stators. However, the rotational movements inherent in the operation of turbine rotors have imposed certain limitations on the configuration and effectiveness of the seals for rotors.
In view of the above, it is an object of the subject invention to improve the efficiency of gas turbine engines.
It is another object of the subject invention to provide improved efficiency for gas turbine engines used in ground vehicles.
Another object of the subject invention is to provide a seal for reducing tip leakage in gas turbine engines.
An additional object of the subject invention is to provide an improved seal for variable stators of a gas turbine engine.
Still another object of the subject invention is to provide a seal for mounting adjacent the radially inner and/or the radially outer ends of the blades of a variable stator in a gas turbine engine.